Conventionally, tissue specimens are prepared for microtome sectioning in two sequential stages. In the first stage the tissue specimen is preserved by chemical fixation and the impregnation of paraffin. The specimen is typically held in a cassette to maintain control and segregate it from other samples. The specimen is placed in a fixation solution, typically neutral buffered formalin, to preserve the chemical structure of the tissue. The specimen is then subjected to a series of solvents, typically alcohols and xylene followed by molten paraffin. The solvents remove the water content of the tissue, and provide a bridge to allow the molten paraffin to penetrate the sample and structurally support it. At the end of the first stage the tissue specimen is loose in the cassette, chemically fixed and impregnated with paraffin. In the second stage the tissue specimen is oriented and embedded in a paraffin block in preparation for sectioning. The specimen is placed and oriented in the bottom of a mold and molten paraffin is poured over the specimen. A jig or fixture for attachment to the microtome is placed in the top of the mold and additional molten paraffin is poured to embed this fixture to the paraffin block. The paraffin block is allowed to cool and solidify. The paraffin block is then removed from the mold, exposing the tissue specimen, and then mounted in the clamp of a microtome for sectioning of the specimen.
It is important that the specimen is accurately positioned in the embedding mold prior to the paraffin embedding step, so that sectioning of the specimen occurs along appropriate planes to reveal the desired cell structure. Presently, accurate positioning of the specimen is achieved by setting the specimen in a desired position in the embedding mold, and allowing a few drops of molten paraffin wax to fall on the specimen to set the specimen in the desired position. Alternatively, a few drops of molten paraffin wax are placed in the bottom of the embedding mold, and the specimen is set in desired position in the still molten paraffin wax. The molten paraffin wax is allowed to cool and solidify. The solidified paraffin wax holds the specimen in the required position through the second stage, after which more molten paraffin wax is added to the cassette or mold to fully embed the specimen. In place of paraffin wax, molten nitrocellulose, gelatin, and various resins have been used to embed the specimen in a desired orientation through the first stage.
Various attempts to improve the preparation of tissue samples for sectioning primarily have involved modifications to the molds or cassettes. See, for example, U.S. Pat. Nos. 3,982,862; 4,557,903; 4,801,553; 5,080,869; and 6,017,476, which are exemplary.